1. Field of the Invention
The present invention relates to a push-buttom switch for use on the keyboard of an electronic equipment and, more particularly, to improvements in a push-button switch capable of making the operator detect the action of the contact thereof by the sense of touch.
2. Description of the Prior Art
Japanese Patent Laid-open Publication No. 60-81719 (U.S. Ser. No. 538,728: U.S. Pat. No. 4,528,431) discloses a push-button switch having a fixed contact point provided on a substrate, and a movable contact point operated by an operating key to open or close the switch, and capable of making the operator detect the action of the movable contact point by the sense of touch. The construction of this known push-button switch will be described hereinafter with reference to FIGS. 13, 14 and 15.
A frame 112 having a cylindrical supporting part 111 is attached to a substrate 114. A key 110 is mounted on the cylinderical supporting part 111 for sliding motion along the cylindrical supporting part 111. The key 110 has a stem 116 extended downward through the interior of the cylindrical supporting part 111. The free end of the stem 116 is bifurcated to form a skirt 117, and a projection 126 is formed on the outer surface of the extremity of the skirt 117. The projection 126 engages a shoulder 127 formed on the inner surface of the cylindrical supporting part 111 to limit the upward movement of the key 110.
A membrane switch 115 is provided on the substrate 114. An actuator 119 of a swing type is provided in place on the upper surface of the membrane switch 115. When the key 110 is depressed, the actuator 119 closes one of the contacts 120 of the membrane switch 115.
A compression coil spring 118 is provided between the key 110 and the actuator 119 with one end thereof seated on a spring seat 121 formed on the stem 116 of the key 110 and the other end thereof fitted on a stud 123 formed integrally with the actuator 119. The compression coil spring 118 is bent slightly in a predetermined direction. The lateral bend of the compression coil spring 118 is limited by the skirt 117 of the stem 116. The actuator 119 has a supporting portion 122 which serves as a pivot in the initial stage of turning motion of the actuator 119, and a projecting portion 124 having a downward convex curved bottom surface. As shown in FIG. 13, a small gap is formed between the projecting portion 124 and the upper surface of the membrane switch 115 at a position corresponding to the contact 120 when the key 110 is not depressed. When the key 110 is depressed to a position shown in FIG. 14, a force acting through the compression coil spring 118 on the actuator 119 increases.
FIG. 15 shows the relation between the pressure applied to the key 110 and the displacement of the key 110. When the key 110 is depressed, a force greater than a force acting on the actuator 119 while the actuator 119 is at an idle position acts on the actuator 119, and a clockwise moment resulting from the bending of the compression coil spring 118 exceeds a counterclockwise moment acting on the actuator 119 while the actuator 119 is at the idle position. Consequently, the actuator 119 turns clockwise on the extremity of the supporting portion 122 thereof, so that the projecting portion 124 of the actuator 119 presses the upper surface of the membrane switch 115 at the position corresponding to the contact 120 to close the contact 120 as shown in FIG. 14. In FIG. 15, a point 131 indicates the status of the key 110 when the contact 120 is closed. When the contact 120 is thus closed, the compression coil spring 118 is bent greatly as shown in FIG. 14, which is detected by the opertor by the sense of touch. Upon the detection of such a mode of bending of the compression coil spring 118, the operator releases the key 110. In an initial period after the key 110 has been released, the actuator 119 continues to turn clockwise because the decreasing rate of the pressure acting on the actuator 119 is greater than the decreasing rate of the clockwise moment acting on the actuator 119 and hence the contact 120 remains closed. Since the compression coil spring 118 tends to recoil, the clockwise moment decreases. A point 132 in FIG. 15 indicates the status of the key 110 under such a condition. Shortly, the counterclockwise moment acting on the actuator 119 exceeds the clockwise moment acting on the actuator 119, and then the actuator 119 starts turning counterclockwise to the idle position. A point 133 in FIG. 15 indicates the status of the key 110 under a condition where the key 110 has returned to its initial position and the actuator 119 is at the position shown in FIG. 13. The deviation of the position of the key 110 in the state indicated by the point 133 from the position of the same in a status indicated by a point 130 is zero. However, the force acting on the key 110 in the status indicated by the point 133 is different slightly from that acting on the same in the status indicated by the point 130 due to frictional resistance against the movement of the key 110.
As is obvious from FIG. 15, the physical hysteresis in the motion of the key 110 indicates that the switching operation of the contact 120 gives a responsive effect, which enables the operator to detect the switching operation of the contact 120. That is, the switching operation of the contact 120 causes a slight movement of the key 110, which can be detected by the operator by the sense of touch.
However, the foregoing known push-button switch has drawbacks that actuators of a complicated shape and many parts are necessary to enable the operator to detect the switching operation of the contacts by the sense of touch, the keys and the actuators must be inserted respectively in different directions increasing the steps of assembling work in assembling the push-button switch, and hence the push-button switch is expensive. Furthermore, the substrate must be removed even in replacing a single faulty actuator with a new one for repair, which requires much time and labor.